This invention relates to pressure-sensitive adhesive-backed sheet materials and, more particularly, to non-curling pressure-sensitive adhesive-backed sheet materials which are temporarily adhered to a release liner.
Adhesive-backed printed or printable sheet products have been in general use for home and business applications for many years. Among the many products made from adhesive-backed sheet stock are labels, name tags, membership tags, etc. These products have become more versatile and easier to use due to the development of pressure-sensitive adhesives. Pressure-sensitive adhesives have the advantage over other types of adhesives of not requiring wetting or heating for activation. It is only necessary to place labels having pressure-sensitive adhesive-coated backs in the desired position on the item to which the label is to be affixed and apply light pressure to bond the label to the item.
Pressure-sensitive adhesive-backed sheet products usually have a temporary, non-adhering protective backing sheet over the adhesive, which enables the user to store and handle the adhesive-coated sheet products without the risk of their accidentally adhering to a surface. The backing sheet, commonly referred to as a release liner, is made from a sheet material, such as paper, and it generally has a release agent, such as silicone, coated onto one of its surfaces The release agent enables the liner to be easily peeled from the label, but also permits the release liner to adhere lightly to the adhesive layer on the back of the label.
The label itself may be made from various materials. For some applications it is desired to make the label from paper because of its low cost. In other applications it is desired to use non-paper substances, such as polymeric sheet material Such materials are often preferred because of their hard surface and lack of sensitivity to moisture. In some instances the label and the release liner are both made from paper. In other instances the label is made from one material, such as polyvinyl chloride, and the release liner is made from a different material, such as paper.
One of the main advantages of pressure-sensitive adhesive-coated labels with release liners is that the labels can be written on or typed on to record information on the label prior to attaching it to a package, file or other item. Information can also be recorded on such labels xerographically by means of laser printers or copying machines Recording information on labels by means of laser printers or copying machines presents no difficulty with respect to curling when the label and the release liner are made from the same material. However, it has been discovered that, if the label is made of polymeric material and the release liner is made of paper, laser printing or copier recording on the label can cause the label to curl. It has been determined that the heat generated by the fusion section of laser printers and copiers causes the curling. It appears that the heat drives moisture from the release liner paper and causes the paper to shrink. The label, on the other hand, is not sensitive to moisture loss and it does not undergo appreciable shrinkage. The net result is that the label tends to curl around the release liner. This is undesirable because it makes the label unattractive and difficult to use.
Prior attempts to eliminate or reduce curling in pressure-sensitive adhesive-backed labels with plastic face sheets and paper-based release liners have led to the development of release liners comprised of a sheet of paper sandwiched between two layers of polyethylene film, with one of the polyethylene layers having a release agent coated on its outer surface. Polyethylene film-covered release liners do not undergo appreciable shrinkage but they suffer from other shortcomings. One shortcoming is that polyethylene film-covered release liners have a tendency to acquire a static electric charge. The static charge prevents the labels from feeding smoothly into laser printer and copying machines.
The problem of static charge build-up on labels with polyethylene film-covered release liners has been partially eliminated by applying a thin layer of tissue paper to the polyethylene layer which does not have the release agent deposited on it. This partial solution has, however, introduced additional problems. For instance, these composite labels are expensive and are often too thick to feed through laser printers and copiers. Furthermore, the tissue itself may swell or shrink from the absorption or loss of moisture.
Another effort to eliminate the problem of curl in composite labels has led to the use of embossed paper as the substrate for the release liner. Embossed paper has less propensity to shrink or swell from moisture loss or gain, apparently because the embossing compensates for the shrinkage or swelling of the paper. Labels with embossed release liners may be less likely to exhibit curling upon being subjected to elevated temperatures but they are not suitable for all uses. The embossed surface of the release liner is often imprinted on the face sheet of the label when the two are pressed together during printing. This problem is particularly acute when the face sheet is made from soft plastic films because these materials tend to conform to the embossed pattern.
The problem of shrinkage due to moisture loss may discourage the use of paper in the manufacture of release liners for plastic laser-printable labels; however, the low cost of paper-based release liners makes their use attractive. Thus, there is a continuing effort to find ways to eliminate shrinkage, swelling and curling problems associated with the use of these liners without introducing other problems.